Typically, public radio-frequency (RF) communications are transmitted at a preconfigured frequency so that a receiver can tune to the particular frequency and receive the communications. In contrast, private communications and military RF systems are transmitted across multiple frequencies (e.g., using frequency hopping and/or spread spectrum techniques) in a short time window. In some instances, these private communications need to be captured by unintended receivers, e.g., law enforcement agencies, military organizations and the like. However, difficulties arise when a communication is transmitted across various frequencies, i.e., frequency hopping is employed, in the form of short RF pulses where each broadcast is on a different frequency.
Without knowing the frequency hopping pattern, a receiver must attempt to capture all signals in the relevant band. Typically, all the signals within the band are digitized and then processed using a very high speed digital signal processing (DSP) system. Such high speed DSP systems are very costly to manufacture, operate and maintain. In some instances, the band of interest is divided into sub-bands and each sub-band is digitized and processed in a corresponding DSP. Such sub-band channelization enables many signals to be quickly processed in parallel using less expensive DSP circuits (i.e., lower speed circuits). However, even a channelized, broad band receiver is very expensive to manufacture, operate and maintain.
Recently, optical systems have found use in broad band signal processing wherein the received RF signals are used to modulate a light signal and the light signal is processed in an optical signal processing section of the receiver. Such techniques, unfortunately, are prone to resonant noise and system instability due to the use of amplifiers within the optical signal processing section.
Therefore, there is a need in the art for an improved method and apparatus for analyzing the spectrum of radio-frequency signals using a fiber optic recirculation loop.